Active resistance summer for a transformer hybrid

ABSTRACT

An electrical circuit in a communications channel includes a first sub-circuit having a first input which receives a composite signal that includes a transmission signal component and a receive signal component, a second input which receives a replica transmission signal, a third input which receives an analog baseline correction current, and an output which provides a receive signal which comprises the composite signal minus the replica signal. A second sub-circuit for controls the analog baseline correction current, so that the magnitude of the composite signal does not exceed a predetermined value of an operating parameter of the electrical circuit. The composite signal, the replica transmission signal, and the analog baseline correction current are directly connected together at a common node of the first sub-circuit.

CROSS-REFERENCE TO RELATED CASES

This application is a continuation-in-part of and claims benefit of priority of U.S. patent application Ser. No. 09/629,092, now U.S. Pat. No. 6,775,529, entitled “ACTIVE RESISTIVE SUMMER FOR A TRANSFORMER HYBRID,” by the same inventors, filed Jul. 31, 2000, the disclosure of which is hereby incorporated by reference. This application is also related to U.S. patent application Ser. No. 09/920,241, now U.S. Pat. No. 7,433,665 entitled “APPARATUS AND METHOD FOR CONVERTING SINGLE-ENDED SIGNALS TO A DIFFERENTIAL SIGNAL, AND TRANSCEIVER EMPLOYING SAME,” by Pierte Roo, filing date of Aug. 1, 2001, the disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to transmitting and receiving electrical signals through communication channels, such as a gigabit channel. In particular, the present invention relates to a transmit canceller that removes transmit signals from receive signals in such communication channels.

2. Background and Related Art

A gigabit channel is a communications channel with a total data throughput of one gigabit per second. A gigabit channel typically includes four (4) unshielded twisted pairs (hereinafter “UTP”) of cables (e.g., category 5 cables) to achieve this data rate. IEEE Standard 802.3ab, herein incorporated by reference, specifies the physical layer parameters for a 1000BASE-T channel (e.g., a gigabit channel).

As will be appreciated by those skilled in the art, a UTP becomes a transmission line when transmitting high frequency signals. A transmission line can be modeled as a network of inductors, capacitors and resistors, as shown in FIG. 1. With reference to FIG. 1, G is normally zero and R(ω R(ω)=k _(R)(1+j)√{square root over (ω)},  (1) where k_(R) is a function of the conductor diameter, permeability, and conductivity. The characteristic impedance of the line is defined by:

$\begin{matrix} {{Z_{0} = \sqrt{\frac{{R(\omega)} + {{j\omega}\; L}}{G + {{j\omega}\; C}}}},} & (2) \end{matrix}$ and at high frequencies, Z₀ becomes approximately √{square root over (L/C)} or approximately 100 ohms in a typical configuration. When properly terminated, a UTP of length d has a transfer function H that is a function of both length (d) and frequency (ω): H(d,ω)=e ^(th(ω)),  (3) where γω=√{square root over ((R(ω)+jωL)(G+jωC))}{square root over ((R(ω)+jωL)(G+jωC))},  (4) and substituting Equations 1 and 4 into Equation 3, and simplifying, approximately yields:

$\begin{matrix} {{H\left( {d,\omega} \right)} \approx {\exp{\left\{ {d\left\lbrack {{\frac{k_{\kappa}}{2}\sqrt{\frac{\omega\; L}{C}}} + {j\left( {{\omega\sqrt{LC}} + {\frac{k_{\kappa}}{2}\sqrt{\frac{\omega\; L}{C}}}} \right)}} \right\rbrack} \right\}.}}} & (5) \end{matrix}$ Equation 5 shows that attenuation and delay are a function of the cable length d.

A transmission path for a UTP typically includes a twisted pair of cables that are coupled to transformers at both a near and far end, as shown in FIG. 2. A transceiver at each end of the transmission path transmits and receives via the same twisted pair. A cable typically includes two patch cords totaling less than 10 m, and a main section of 100 m or even longer. The transmitters shown in FIG. 2 are modeled as current sources. The near end current source supplies a current I_(tx). The near end transmit voltage (e.g., I_(tx)R_(tx)) is detected and measured across resistor R_(tx). A receive signal V_(rcv) (e.g., a signal transmitted from the far-end transceiver) is also detected and measured across resistor R_(tx). Hence, V_(tx) includes both transmit (I_(tx)R_(tx)) and receive (V_(rcv)) signals. Accordingly, the signal V_(rcv) (e.g., the signal from Transceiver B) received at Transceiver A can be obtained by taking the difference between the transmit voltage and the measured voltage V_(tx), as follows: V _(rcv) =V _(tx) −I _(tx) R _(tx).  (6)

Conventional solutions for removing transmit signals from receive signals often employ known transconductor (“Gm”) summing stages or other current based methods. As will be appreciated, these methods often introduce signal distortion into the receive signal. Also, some transconductors have a limited signal dynamic range. Accordingly, conventional methods are often inadequate for applications requiring signal recovery. Additionally, known summing circuits, such as weighted summers using operational amplifiers, have not heretofore been modified to accommodate the intricacies associated with canceling transmit signals or regulating baseline wander (described below). A known weighted summer is discussed in Chapter 2 of “Microelectronic Circuits, Third Edition,” by A. S. Sedra and K. C. Smith, 1991, incorporated herein by reference.

As will be appreciated by those skilled in the art, the receive signal V_(rcv) typically contains additional components, due to baseline wander, echoes and crosstalk, for example.

Baseline wander is preferably corrected for when transmitting and receiving signals over transmission lines. Removing DC components from a receive signal using transformer coupling can cause baseline wander. As will be appreciated by those skilled in the art, baseline wander represents a deviation from an initial DC potential of a signal.

“Echoes” typically represent a residual transmit signal caused by reflections that appear in the receive signal. Echoes can cause undue interference depending on the size of the reflection.

Capacitive coupling between the channels, as shown in FIG. 3, causes crosstalk. Four channels TX1-TX4 are shown in FIG. 3. The capacitive coupling between TX1 and each of TX2, TX3 and TX4 are modeled by capacitors C₁₋₂, C₁₋₃, C₁₋₄, respectively. The capacitive coupling forms a high-pass filter between channels and therefore crosstalk contains mostly high frequency components. As will be appreciated by those skilled in the art, normally only the near-end crosstalk (NEXT) needs to be considered, since crosstalk is usually small and the transmission line provides further attenuation of the far-end crosstalk (FEXT).

Accordingly, there are many signal-to-noise problems to be solved in the art. Hence, an efficient transmission canceller is needed to remove a transmit signal from a receive signal without introducing excess signal distortion. An electrical circuit is also needed to subtract a transmit signal from a receive signal. There is a further need of an electrical circuit to correct baseline wander.

SUMMARY OF THE INVENTION

The present invention relates to a transmit signal canceller for use in a transformer hybrid. Such a hybrid includes a junction for transmitting and receiving signals. In the present invention, an active resistive summer can be used to cancel a transmit signal from a receive signal.

According to the invention, an electrical circuit in a communications channel is provided. The electrical circuit includes an active resistive summer having: (i) an input for a composite signal, the composite signal including a transmission signal component and a receive signal component, (ii) an input for a replica transmission signal, and (iii) an output for a receive signal which includes the composite signal minus the replica signal.

According to an another aspect of the present invention, a transmit signal canceller in a communication channel is provided. The channel includes a first transceiver for transmitting and receiving signals and a replica transmitter for generating a replica transmission signal input. A composite signal at a near end includes a transmission signal of the first transceiver and a received signal of a second transceiver. The transmit canceller includes: (i) an operational amplifier having a positive input terminal, a negative input terminal, and an output terminal; (ii) a feedback element in communication with the negative input terminal and the output terminal; (iii) a first input resistor in communication with the negative input terminal and the measured signal input; (iv) a second input resistor in communication with the negative input terminal and the replica signal input; and (v) a predetermined voltage source in communication with the positive terminal of the operational amplifier. The receive signal is an output at the output terminal of the operational amplifier.

According to still another aspect of the present invention, a communication system including a first transmission channel with a first end and a second end is provided. The first end couples to a first transformer and the second end couples to a second transformer. A first transceiver transmits and receives signals via the first transformer and a second transceiver transmits and receives signals via the second transformer. A first signal is supplied at the near end. The first signal includes a transmission signal component of the first transceiver and a receive signal component of the second transceiver. The communications system includes: (i) a replica transmitter that generates a replica of the transmission signal component of the first transceiver; (ii) a filter to filter the replica signal; (iii) an active resistive summer receiving the first signal, and the filtered replica signal as inputs to reduce the transmission signal component at an output of the active resistive summer.

According to still another aspect of the present invention, a method of correcting baseline wander in a receive signal in a communications channel having a near and far end is provided. The channel includes a first transceiver at the near end and a second transceiver at the far end, each to transmit and receive signals. The method includes the steps of: (i) providing a composite signal, the composite signal including a transmission signal of the first transceiver and a receive signal of the second transceiver; (ii) generating a replica of the transmission signal; (iii) subtracting the replica signal from the composite signal through an active resistive summer; and (iv) providing a baseline correction current into the active resistive summer.

According to still another aspect of the present invention, an electrical circuit in a communications system is provided. A composite signal including a transmission signal component and a receive signal component, a replica transmission signal and a common-mode shift current are provided. Further circuitry is provided to control the magnitude of the common-mode shift current so that the magnitude of the composite signal does not exceed a predetermined value of an operating parameter of the electrical circuit.

In still another aspect of the present invention, an electrical circuit in a communications system is provided. An active resistive summing circuit produces a receive signal as a difference between a composite signal and a replica transmission signal, the composite signal comprising a transmission signal component and a receive signal component. Further circuitry is provided which controls the magnitude of the composite signal.

In still another aspect of the present invention, another electrical circuit in a communications system is provided. An active resistive summer is provided that receives a composite signal that includes a transmission signal component and a receive signal component, a replica transmission signal, and a common-mode shift current signal. The active resistive summer provides an output which is a receive signal that comprises the composite signal minus the replica signal. Further circuitry is provided which controls the magnitude of the common-mode shift current to thereby control the magnitude of the composite signal.

These and other objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of the present invention will be more readily understood from a detailed description of the preferred embodiments taken in conjunction with the following figures.

FIG. 1 is a circuit diagram illustrating a transmission line model.

FIG. 2 is a circuit diagram illustrating a transmission path across a twisted pair of cables, the cables being coupled to transformers at each end.

FIG. 3 is a diagram-illustrating crosstalk between channels in a gigabit channel.

FIG. 4 is a block diagram illustrating a system overview of a communications channel.

FIG. 5 is a circuit diagram illustrating a transmitter.

FIG. 6 is a graph illustrating a transmit signal.

FIG. 7 is a graph illustrating a composite signal with echoes.

FIG. 8 is a circuit diagram illustrating a replica transmitter.

FIG. 9 is a graph illustrating a receive signal.

FIG. 10 is a block diagram illustrating a low-pass filter.

FIG. 11 is a circuit diagram illustrating an active resistive summer.

FIG. 12 is a circuit diagram illustrating an error detection circuit.

FIG. 13 is a circuit diagram illustrating a low-pass filter.

FIG. 14 is a circuit diagram illustrating a conventional voltage controlled current source.

FIG. 15 is a circuit diagram illustrating one exemplary embodiment of a common-mode feedback circuit coupled with a transmitter.

FIG. 16 is a circuit diagram illustrating one exemplary implementation of an operational amplifier utilized in the common-mode feedback circuit.

FIG. 17 is a circuit diagram illustrating one exemplary embodiment of a common-mode shift current control circuit coupled with a transmitter.

FIG. 18 is a circuit diagram illustrating one exemplary embodiment of a common-mode shift control circuit coupled with a transmitter.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The preferred embodiments will be described with respect to a gigabit channel, as used, for example, in an Ethernet network; and to electrical circuits associated with separating transmit and receive signals in such a gigabit channel. The preferred embodiments will also be described with respect to baseline wander correction in such a gigabit channel. However, as will be appreciated by those skilled in the art, the present invention is also applicable to other transmission channels, and to other electrical circuits having applications requiring cancellation of transmit signals, for example.

FIG. 4 is a block diagram illustrating principle components for one of the four channels in a preferred gigabit channel configuration for use in an Ethernet network. As illustrated in FIG. 4, a vertical dashed line divides analog and digital processing components. The analog components preferably include a transmitter (“XMTR”) 1, replica transmitter (“Replica XMTR”) 2, transmit canceller 3, baseline correction module 4, low pass filter (“LPF”) 5, analog-to-digital converter (“ADC”) 6, and phase-lock loop (“PLL”) 7. A known PLL can be used with the present invention.

Digital processing components preferably include a transmitter encoder 10, echo module 11, NEXT cancellers 12-14 to assist in removing echoes, synchronization module 15, FIR (Finite Impulse Response) equalizer 16 and a DFE (Decision Feedback Equalizer) 17 to equalize a receive signal, and a Viterbi module 18. The digital processing components also include baseline correction modules 19 and 20 to correct residual baseline wander. A timing recovery module 21, an error correction detector 22 (described in further detail below), and summing junction 23 are also shown. The individual digital components designated by blocks in FIG. 4 are all well known in the communication arts, and their specific construction and operation are not critical to the operation or best mode for carrying out the present invention.

The analog “front-end” components shown in FIG. 4 will now be described in even further detail. The front-end analog components are preferably designed and constructed via customized integrated circuits. However, as will be appreciated by those skilled in the art, the inventive circuits and corresponding configuration could also be realized using discrete components as well.

As illustrated in FIG. 5, transmitter 1 preferably includes a current-source I_(tx) that generates a transmit signal over a resistor R_(tx). An appropriate value for resistor R_(tx) can be selected to match the line impedance, for example. In one preferred embodiment, a resistor center tap is set to 2.5 volts so the transmitter 1 effectively sees a differential impedance of 25 ohms. Preferred performance specifications for the transmitter 1 are further detailed in Table 1, below.

An impulse transmit signal can be generated from a unit square pulse of 1 T width filtered by a one-pole, low-pass filter (not shown) with a cutoff frequency between 85 MHz and 125 MHz. Slew-rate control can also be used to limit the rise and fall times and thus reduce the high frequency components of a transmit signal. Of course, any transmit signal preferably fits into the transmit template provided by the IEEE 802.3ab Standard. An ideal transmit pulse is shown in FIG. 6.

A measured voltage V_(tx) across R_(tx) (FIG. 5) is shown in FIG. 7. The measured signal V_(tx) contains interference caused by line reflections (e.g., echoes). The reflections are caused by impedance discontinuity due to impedance mismatch between different cables. For example, a large reflection pulse at 60 ns as shown in FIG. 7 corresponds to a reflection from the impedance discontinuity at an adapter connecting a 5 m patch cord to a 100 m cable. The magnitude of the echoes can be significant when compared to the magnitude of the receive signal at a long line length, and therefore, echo cancellation, as provided by the NEXT cancellers 12-14 shown in FIG. 4, is employed.

A receive signal V_(rcv) (e.g., a signal received from a far-end transceiver) is also measured across resistor R_(tx), as shown in FIG. 5. Accordingly, the near end transmit signal (I_(tx)R_(tx)) is preferably canceled or reduced from the composite signal V_(tx) in order to effectively recover the far-end received signal V_(rcv). This type of active cancellation can be accomplished with a replica transmit signal V_(txf). Accordingly, a replica transmitter 2 (to be described below) is provided to generate a signal V_(txr) to be subtracted from the measured signal V_(tx), thus, effectively reducing the transmit signal (I_(tx)R_(tx)).

A receive signal x(t) transmitted with pulse amplitude modulation (“PAM”) is define by:

x ⁡ ( t ) = ∑ n = 1 ⁢ ⁢ a n ⁢ p ⁡ ( i - nT ) , ( 7 ) where a_(n) is the transmit symbols and p(t) is the channel pulse derived by convoluting an impulse transmit pulse with a channel response defined by Equation 5. The receive signal for a 100 m cable is heavily attenuated by the transmission line and the pulse width is dispersed, as shown in FIG. 9. A 100 m UTP delays the signal by about 550 ns. Signal equalization preferably uses high frequency boosting via the FIR 16 to remove precursor intersymbol interference (“ISI”) and to insert a zero crossing for timing recovery 21. The DFE 17 is used to remove postcursor ISI.

The receive signal's elongated tail results from transformer coupling (e.g., a high-pass filter) with a time constant (e.g., L/R) typically on the order of micro-seconds. Since the receive signal contains little or no average DC energy, the negative tail has the same amount of energy as the positive pulse. In this regard, the signal's area integral is zero. In a typical example, a tail can last over 10 μs with a magnitude of no more than 0.5 mV. The long tail causes any DC bias to drift back toward zero, which can lead to baseline wander. As will be appreciated, this response time is too long to be practically removed by a digital equalizer, but the response is slow enough to be cancelled using a slow integrator, for example. The baseline wander canceller 4 is preferably decision directed to minimize the error defined by the difference between the equalized value and it's sliced value, as discussed below.

As illustrated in FIG. 8, the replica transmitter 2 includes a current source I_(txr). I_(txr) is coupled to a voltage V through resistors R, as shown in FIG. 8. In a preferred embodiment, R is 100 ohms and V is about 2.5 volts. The replica signal V_(txr) is preferably filtered through a known low-pass filter to obtain a low-pass replica signal (“V_(txrl)”), as shown in FIG. 10. Replica signal V_(txr) can also be inverted in a known manner to produce −V_(txr). The preferred performance specifications for the transmitter 1 and replica transmitter 2 are shown in Table 1.

TABLE 1 Transmitter and Replica Performance Specifications Parameters Specifications Transmit Current +/−40 mA Replica Transmit Current ¼ of transmit current Number of levels 16 (not including 0) Number of sub-units 8 (sequentially delayed) Transmit Profile [1 1 2 2 1 1], w/~1 ns delay Replica Transmit Profile [1 1 3 3], w/~1 ns delay R_(tx) 100Ω

A transmit signal canceller 4 is illustrated in FIG. 11. The transmit canceller 4 removes the transmission signal (I_(tx)R_(tx)) from the measured (or detected) transmit V_(tx) signal. In particular, the transmit canceller includes an active resistive summer that provides a large input dynamic range and stable linearity characteristics, while removing (e.g., reducing or canceling) the unwanted transmit signal component.

As illustrated in FIG. 11, the active summer includes an operational amplifier (“op-amp”) with inverting feedback. The op-amp is preferably constructed using integrated circuits in a known manner. The summer receives V_(txrl), V_(tx), −V_(txr), I_(cms), and I_(bl) as input signals. I_(bl) is a baseline wander control current, and I_(cms) is a common-mode shift current, each as further discussed below.

As will be appreciated by those skilled in the art, a transformer typically has high-pass characteristics. Accordingly, replica signal −V_(txr) is combined (e.g., subtracted via the active resistive summer) with the low pass replica signal V_(txrl) to produce a high-pass replica signal. As an alternative configuration, V_(txr) could be filtered through a known high-pass filter prior to the transmit canceller 3 stage.

Returning to FIG. 11, receive signal V_(rcv) is determined from the following relationships.

Let:

-   -   Vi=voltage for the op-amp's positive terminal;     -   V₁=V_(txrl);     -   V₂=V_(tx);     -   −V₃=−V_(txr);     -   i₄=I_(cms); and     -   i₅=I_(bl).

Then: i ₁ +i ₂ −i ₃ −i ₄ −i ₅ =i ₀; and

$\begin{matrix} {{{\frac{V_{1} - {Vi}}{R_{1}} = i_{1}};{\frac{V_{1} - {Vi}}{R_{1}} = i_{2}};{\frac{{{Vi}--}V_{1}}{R_{1}} = i_{3}};{\frac{{Vi} - {Vrcv}}{R_{F}} = {{{i_{0}.\frac{V_{1} - {Vi}}{R_{1}}} + \frac{V_{1} - {Vi}}{R_{1}} - \frac{{Vi} + V_{3}}{R_{1}} - i_{4} - i_{5}} = \frac{{Vi} - {Vrcv}}{R_{F}}}}}{{\frac{V_{1} + V_{2} - {V_{3}3{Vi}}}{R_{1}} - i_{1} - i_{5}} = \frac{{Vi} - {Vrcv}}{R_{r}}}{{{\frac{R_{r}}{R_{1}}\left( {V_{1} + V_{2} - V_{3} - {3{Vi}}} \right)} - {i_{4} \cdot R_{r}} - {i_{3} \cdot R_{F}}} = {{Vi} - {Vrcv}}}{{{\frac{R_{F}}{R_{1}}\left( {V_{1} + V_{2} - V_{3} - {3{vi}}} \right)} - {R_{r}i_{4}} - {R_{r}i}},{{- {Vi}} = {- {Vrcv}}}}{V_{rcv} = {V_{1} - {\frac{R_{r}}{R_{1}}\left( {V_{1} + V_{2} - V_{3} - {3{Vi}}} \right)} + {R_{r}\left( {i_{1} + i_{3}} \right)}}}} & (8) \end{matrix}$

Substituting the input signals for their placeholders yields the definition for V_(rcv), as follows:

$\begin{matrix} {V_{rcv} = {{Vi} - {\frac{R_{F}}{R_{1}}\left( {{Vtxrl} + {Vtx} - {Vtxr} - {3{Vi}}} \right)} + {{R_{r}\left( {{Icms} + {lbl}} \right)}.}}} & (9) \end{matrix}$ The gain is preferably set between 0.75 and 1 (e.g., R_(F)/R₁ equals 0.75 to 1). For a small signal analysis, Vi can be set to zero (0). Also, as will be appreciated by those skilled in the art, in a fully differential circuit, Vi effectively drops out of the equations since V_(rcv)=V_(rcv) ⁽⁺⁾−V_(rcv) ⁽⁻⁾. As discussed, V_(txrl) and −V_(txr) are combined through the active summer to provide a high-pass replica signal (“V_(txrh)”). The receive signal V_(rcv) can then be recovered as shown by Equation 9.

Preferred transmit canceller specifications are detailed in Table 2, below.

TABLE 2 Transmit Canceller Performance Specifications Parameters Specifications Input Dynamic Range +/−2.5 V(diff.) for transmit signal Output Dynamic Range +/−1 V(diff.) Input impedance High, ~10 k. Output impedance Low Cutoff frequency Greater than 31.5 Mhz DC Gain 0.85 -dependent of the LPF 5 and ADC 6 characteristics (FIG. 4) Power 25 mw, including LPF 5 (FIG. 4) R_(f) 8.5 KΩ; or 7.5 KΩ for increased attenuation Vi 2.0 volts R₁ 10 KΩ

A known current mode circuit, e.g., a voltage controlled current source (VCCS) as shown in FIG. 14, with feedback preferably sets the summer input current-mode voltage (V_(cm)). Of course, other known current mode circuits could be employed with the present invention. This current-mode circuit shifts the common-mode of both the transmit and replica transmit signals. The input to the op amp (V_(aip), V_(ain)) is compared against the desired op amp output common-mode voltage (V_(d)): V _(d)=(V _(aip) −V _(cm))+(V _(ain) −V _(cm)).  (10)

Then, the common-mode shift current can be determined from: I _(cms) =V _(d) g _(m) +I _(o),  (11) where g_(m) is a transconductance and I_(o) is an offset current. An appropriate transconductance and offset current can be selected by setting V_(cm)=I_(cms)R_(F)=V_(d)g_(m)R_(F)+I_(o)R_(F), to ensure a proper common-mode voltage seen by the op amp inputs. In this manner, the common mode shift current I_(cms) can be regulated to pull down the common mode voltage of the operational amplifier as needed.

Baseline wander current I_(bl) is also “summed” by the active resistive summer, as shown in FIG. 11, to correct baseline wander. Approximately ninety percent (90%) of all system baseline correction can be obtained through the active summer. The remaining baseline residual can be digitally corrected through an equalizer, for example. As will be appreciated, the FIG. 11 topology allows the current sources (I_(bl) and I_(cms)) to each have a fixed output voltage, thus, minimizing current deviation due to finite output resistance.

The baseline wander correction module 4 preferably corrects for baseline wander using a decision-directed method, such as a discrete integrator. The decision-directed method can be implemented with a known charge pump, where the pump sign (e.g., +1/−1) is determined digitally using an error between the equalized baseline signal (y_(k)) and a sliced baseline signal (y⁺ _(k)), as shown in FIG. 12. As will be appreciated by those skilled in the art, the expected error value (e.g., E[e_(k)]) is ideally driven to zero. The charge pump is preferably pumped up or down based on the error value. For example, a positive error implies that a negative value should be input into the charge pump. For a negative error, a positive value should be input into the charge pump. The charge pump preferably has at least two current settings to regulate I_(bl). Of course, a charge pump with many current settings could be used to obtain finer baseline correction control.

The preferred baseline wander correction performance specifications are further detailed in Table 3, below.

TABLE 3 Baseline Wander Correction Specification Parameters Specifications Output Dynamic Range +/−100 uA (diff.), (+/−1 V/R₁, R₁ = 10 kΩ) Output impedance High Integration Factors 2 mV/T, 4 mV/T Bandwidth >100 MHz

A second-order low-pass filter, as shown in FIG. 13, is cascaded after the summer to preferably flatten the frequency response out to about 31.25 MHz (<1 dB). A minimum overall attenuation of 20 dB at 125 MHz is desirable for the low pass filter. In a sampled system, some aliasing beyond Nyquist frequency (or excess bandwidth) is acceptable, but minimum aliasing is allowed at the sampling frequency. The transmitted data is preferably band-limited to the Nyquist rate.

Preferred performance characteristics of the low pass filter 5 are further detailed in Table 4, below.

TABLE 4 LPF Performance Specification Parameters Specifications Input Dynamic Range +/−1 V (diff.) Output Dynamic Range +/−1 V (diff.) Input impedance High, ~10 k. Output impedance Low Cutoff frequency 50-60 Mhz. Q (2nd order) ~1 Input impedance High, ~10 k. Output impedance Low, <100 DC gain 1

As an alternative arrangement, a third-order Sallen and Key low pass filter as disclosed in a co-pending application by the same inventor of this application, titled “CALIBRATION CIRCUIT,” filed concurrently herewith, and hereby incorporated by reference, could be used as filter 5. Similarly, the calibration circuit disclosed therein could also be used to calibrate the low pass filter 5.

Analog-to-digital converters are well know in the art. As will be appreciated, the ADC 6 resolution is often determined by system digital processing requirements. In a preferred embodiment, the Viterbi detector 18 requires an effective 7-bit resolution. Residual baseline wander, echoes, and crosstalk increase the dynamic range by about 200-300 mV, which increases the required resolution. The reduction in dynamic range due to insertion loss for a 100 m cable is approximately 40%. Accordingly, an 8-bit resolution is preferred.

The preferred ADC performance specifications are further detailed in Table 5, below.

TABLE 5 ADC Performance Specification Parameters Specifications Resolution 8-bits minimum. Sampling frequency 8 MHz (125 MS) Source Output Impedance Low, ~200-400Ω

As previously discussed, the transmitter 1 preferably includes a current-source I_(tx) that a resistor R_(tx) to generate a transmit signal voltage equal to I_(tx)R_(tx). In one preferred embodiment, a center tap of a transformer is connected to a 2.5 volt supply voltage. If the transmitter 1, for example, transmitting 1 volt or receiving 1 volt, then the possible swing in voltage across resistor R_(tx) is 2 volts. More specifically, the voltage across resistor R_(tx) can vary between 1.5 volts and 3.5 volts. If the power supply voltage source for the circuit is less than the maximum voltage that could be present across resistor R_(tx), then improper operation or damage can result. It is therefore preferable to provide common-mode shift current control circuitry which will control the amount of current being drawn across resistor R_(tx) so as to keep the composite voltage signal within an appropriate operating range.

One embodiment of the present invention is depicted in FIG. 15. Common-mode shift current control circuitry is illustrated as common-mode feedback circuitry including an operational amplifier 24 and a pair of field effect transistors (FETs) 25 and 26. The operational amplifier 24 includes a first input terminal, a second input terminal, and an output. The operational amplifier 24 receives a differential voltage signal at the first input terminal, wherein the differential voltage signal is a composite signal which includes components related to the voltage transmitted across the transformer as well as the voltage received across the transformer. A common-mode voltage signal V_(CM) is received at the second input terminal. In the embodiment depicted in FIG. 15, V_(CM)=1.5 volts. The output of the operational amplifier 24 is a common-mode shift current control signal, V_(D).

As will be appreciated by those skilled in the art, when the voltage applied at the first input terminal is different from the voltage applied at the second input terminal, the operational amplifier 24 will sense the inequality. Referring to FIG. 16, the internal operation of the operational amplifier 24 is shown. When the current I_(n) is greater than the current I_(p), the voltages V_(p) and V_(n) will not be in balance around V_(CM). As a result of such an imbalance, V_(D) will increase. Alternatively, when the current I_(p) is greater than the current I_(n), the voltages V_(p) and V_(n) will also not be in balance around V_(CM). As a result of such an imbalance, V_(D) will decrease.

Referring again to FIG. 15, the effects of increasing or decreasing V_(D) are described. By controlling V_(D), the common-mode shift current flowing through the resistors R₁ can be controlled. By controlling the common-mode shift current, V_(tx) is thereby also controlled. By being able to control T_(tx), the situation in which V_(tx) exceeds an operating parameter of the circuit, such as the power supply voltage source, is avoided. Specifically, it is undesirable for the composite voltage signal to exceed the supplied voltage for the circuit. Using the common-mode feedback arrangement depicted in the embodiment of FIG. 15, V_(tx) is controlled in relation to the common-mode voltage signal. I_(p)=I_(n)  (13) V _(p) =V _(txp) −I _(p) R ₁  (14) V _(n) =V _(txp) −I _(n) R ₁  (15)

Referring to Equation 13, it should be appreciated that when the common-mode shift currents I_(p) and I_(n) are equal, then the voltage drop that occurs over the resistor R₁ is the same for both components of the composite differential signal. See Equations 14 and 15. The differential signal applied to the input of the operational amplifier is therefore of the same magnitude as the differential composite signal.

It should be appreciated that the common-mode feedback circuit of FIG. 15 should work regardless of the resistor value chosen, for example 2R₁. Because the current drop across the resistors will be the same in both components of the differential signal, the differential voltage signal applied to the input of the active resistive summer will be of the same magnitude as the composite voltage signal. The distinction relates to the output of the summer, and not to the operation or effectiveness of the common-mode shift current control circuitry.

In another embodiment of the present invention, a constant current source is provided to generate the common-mode shift current control. Referring to FIG. 17, constant current sources 27 and 28 are depicted as controlling the common-mode shift currents I_(p) and I_(n). V _(tx2) =V _(tx) −IR ₂  (16) Because I_(p) and I_(n) are equal, the magnitude of the differential signal applied to the active resistive summer is therefore shown in Equation 16. As compared with the embodiment in which the common-mode feedback circuit is utilized to control the magnitude of the common-mode shift current, and therefore the magnitude of the composite signal relative to the applied common-mode voltage, in the embodiment utilizing constant current sources, the voltage drop between the composite differential signal and the differential signal applied to the summer will be constant, i.e. IR₂.

In another embodiment of the present invention, the common-mode shift current control circuitry includes a resistor divider. Through the use of a resistor divider, the voltage signal applied to the summer is a proportionately reduced signal as compared with the composite voltage signal. V _(tx3) =V _(tx) R ₂/(R ₁ +R ₂) It should be appreciated by those skilled in the art that, by selecting appropriate values for resistors R₁ and R₂, the magnitude of the common-mode shift current and the composite signal can be controlled so that the magnitude of the composite signal does not exceed an operating parameter of the communications circuit, such as the power supply voltage source.

Thus, a transmit canceller including an active resistive summer has been described. Such an active resistive summer has not heretofore been developed for applications such as canceling signals in gigabit channels. Correcting baseline wander through such an active resistive summer has also been described herein. Controlling common-mode shift current has also been described herein.

While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it will be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention covers various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

For example, while preferred circuit configurations and component values have been described, it will be understood that modifications could be made without deviating from the inventive structures. For example, values for the feedback and input resistors R_(f) and R_(i) could be changed to obtain higher or lower gains. Also, an active resistive summer could be constructed to sum only the measured signal V_(tx) and the replica signal V_(txr) (or a high-pass version of the replica), for example. Additionally, while the communication channel has been described with respect to a twisted pair of cables, the invention may also be practiced with other communication channels such as optical and wireless channels. Moreover, this invention should not be limited to gigabit transmission rates and can be practiced at any transmission rate requiring the signal processing characteristics of the invention. Of course, these and other such modifications are covered by the present invention. 

1. An electrical circuit in a communications channel, comprising: a first sub-circuit having a first input which receives a composite signal that includes a transmission signal component and a receive signal component, a second input which receives a replica transmission signal, a third input which receives an analog baseline correction current, and an output which provides a receive signal which comprises the composite signal minus the replica signal; and a second sub-circuit for controlling the analog baseline correction current, so that the magnitude of the composite signal does not exceed a predetermined value of an operating parameter of the electrical circuit, wherein the composite signal, the replica transmission signal, and the analog baseline correction current are directly connected together at a common node of the first sub-circuit.
 2. The electrical circuit of claim 1, further comprising a power supply voltage source of a predetermined magnitude, wherein the operating parameter is the predetermined magnitude of the power supply voltage source.
 3. The electrical circuit of claim 1, wherein the second sub-circuit includes a common-mode feedback circuit.
 4. The electrical circuit of claim 3, wherein the common-mode feedback circuit includes an operational amplifier.
 5. The electrical circuit of claim 4, wherein the operational amplifier has a first input which receives a first differential component of the composite signal, a second input which receives a second differential component of the composite signal, a third input which receives a common-mode voltage signal, and an output which provides a baseline correction current control signal.
 6. The electrical circuit of claim 5, wherein the common-mode feedback circuit further includes a pair of transistors, each transistor having a respective input and wherein the output of the operational amplifier is coupled to the respective input of each of the transistors.
 7. The electrical circuit of claim 1, wherein the second sub-circuit includes a current source.
 8. The electrical circuit of claim 7, wherein the current source provides a constant baseline correction current control signal.
 9. The electrical circuit of claim 1, wherein the second sub-circuit includes a resistor divider.
 10. The electrical circuit of claim 9, wherein the resistor divider comprises a plurality of resistors, each of the resistors having a respective characteristic resistance.
 11. The electrical circuit of claim 10, wherein the resistor divider provides a baseline correction current control signal that is related to the respective resistances of each of the resistors.
 12. An electrical circuit in a communications channel, comprising: an active resistive summing circuit which produces a receive signal as a difference between a composite signal and a replica transmission signal, the composite signal comprising a transmission signal component and a receive signal component; and an analog baseline correction current control circuit which controls the magnitude of the composite signal, wherein the composite signal, the replica transmission signal, and an output of the analog baseline correction current are directly connected together at a common node of the active resistive summing circuit.
 13. The electrical circuit of claim 12, further comprising a power supply voltage source of a predetermined magnitude.
 14. The electrical circuit of claim 13, wherein the analog baseline correction current control circuit controls the magnitude of the composite signal to be less than the magnitude of the power supply voltage source.
 15. The electrical circuit of claim 12, wherein the analog baseline correction current control circuit includes a common-mode feedback circuit.
 16. The electrical circuit of claim 15, wherein the common-mode feedback circuit includes an operational amplifier.
 17. The electrical circuit of claim 16, wherein the operational amplifier has a first input which receives a first differential component of the composite signal, a second input which receives a second differential component of the composite signal, a third input which receives a common-mode voltage signal, and an output which provides a baseline correction current control signal.
 18. The electrical circuit of claim 17, wherein the common-mode feedback circuit further includes a pair of transistors, each transistor having a respective input and wherein the output of the operational amplifier is coupled to the respective input of each of the transistors.
 19. The electrical circuit of claim 12, wherein the analog baseline correction current control circuit includes a current source.
 20. The electrical circuit of claim 19, wherein the current source provides a constant baseline correction current control signal.
 21. The electrical circuit of claim 12, wherein the analog baseline correction current control circuit includes a resistor divider.
 22. The electrical circuit of claim 21, wherein the resistor divider comprises a plurality of resistors, each of the resistors having a respective characteristic resistance.
 23. The electrical circuit of claim 22, wherein the resistor divider provides a current control signal that is related to the respective resistances of each of the resistors.
 24. An electrical circuit in a communications channel, comprising: an active resistive summer having a first input which receives a composite signal that includes a transmission signal component and a receive signal component, a second input which receives a replica transmission signal, a third input which receives an analog baseline correction current, and an output which provides a receive signal which comprises the composite signal minus the replica signal; and a baseline correction current control circuit which controls the magnitude of the analog baseline correction current to thereby control the magnitude of the composite signal, wherein the composite signal, the replica transmission signal, and the analog baseline correction current are directly connected together at a common node of the active resistive summer.
 25. The electrical circuit of claim 24, further comprising a power supply voltage source of a predetermined magnitude.
 26. The electrical circuit of claim 25, wherein the magnitude of the analog baseline correction current is controlled to control the magnitude of the composite signal to be less than the magnitude of the power supply voltage source.
 27. The electrical circuit of claim 25, wherein the magnitude of the analog baseline correction current is controlled to control the magnitude of the composite signal to be equal to the magnitude of the power supply voltage source.
 28. The electrical circuit of claim 24, wherein the baseline correction current circuit includes a common-mode feedback circuit.
 29. The electrical circuit of claim 28, wherein the common-mode feedback circuit includes an operational amplifier.
 30. The electrical circuit of claim 29, wherein the operational amplifier has a first input which receives a first differential component of the composite signal, a second input which receives a second differential component of a composite signal, a third input which receives a common-mode voltage signal, and an output which provides a baseline correction current control signal.
 31. The electrical circuit of claim 30, wherein the common-mode feedback circuit further includes a pair of transistors, each transistor having a respective input and wherein the output of the operational amplifier is coupled to the respective input of each of the transistors.
 32. The electrical circuit of claim 24, wherein the baseline correction current control circuit includes a current source.
 33. The electrical circuit of claim 32, wherein the current source provides a constant baseline correction current control signal.
 34. The electrical circuit of claim 24, wherein the baseline correction current control circuit includes a resistor divider.
 35. The electrical circuit of claim 34, wherein a plurality of resistors, respective characteristic resistance.
 36. The electrical circuit of claim 35, wherein the resistor divider provides a baseline correction current control signal that is related to the respective resistances of each of the resistors.
 37. A communication method for a communications channel, comprising: receiving at a first input a composite signal that includes a transmission signal component and a receive signal component; receiving at a second input a replica transmission signal; receiving at a third input an analog baseline correction current; providing an analog baseline correction current to substantially prevent the magnitude of the composite signal from exceeding a predetermined value of an operating parameter of the electrical circuit; and providing a receive signal comprising the composite signal minus the replica signal at an output, wherein the composite signal, the replica transmission signal, and the analog baseline correction current are directly connected together at a common node.
 38. The method of claim 37, further comprising providing a power supply voltage source of a predetermined magnitude, wherein the operating parameter is the predetermined magnitude of the power supply voltage source.
 39. The method of claim 37, wherein the analog baseline correction current is provided by a common-mode feedback circuit.
 40. The method of claim 39, wherein the common-mode feedback circuit includes an operational amplifier.
 41. The method of claim 40, further comprising: receiving a first differential component of the composite signal at a first input of the operational amplifier; receiving a second differential component of the composite signal at a second input of the operational amplifier; receiving a common-mode voltage signal at a third of the operational amplifier; and providing a baseline correction current control signal at an output of the operational amplifier.
 42. The method of claim 41, further comprising receiving the baseline correction current control signal at an input of a first transistor and at an input of a second transistor.
 43. The method of claim 37, wherein the baseline correction current is provided by a current source.
 44. The method of claim 43, further comprising the step of providing a constant analog baseline correction current.
 45. The method of claim 37, wherein the analog baseline correction current is provided by a resistor divider.
 46. The method of claim 45, wherein the resistor divider comprises a plurality of resistors, each resistor having a respective characteristic resistance.
 47. The method of claim 46, wherein the analog baseline correction current is related to the respective resistances of each of the resistors.
 48. A communication method for a communications channel, comprising: producing a receive signal as a difference between a composite signal and a replica transmission signal, the composite signal comprising a transmission signal component and a receive signal component; controlling the magnitude of the composite signal; receiving at a first input a first differential component of the composite signal; receiving at a second input a second differential component of the composite signal; receiving at a third input a common-mode voltage signal; and providing at an output a baseline correction current control signal to control an analog baseline correction current, wherein the composite signal, the replica transmission signal, and the baseline correction current control signal are directly connected together at a common node.
 49. The method of claim 48, further comprising providing a power supply voltage source of a predetermined magnitude.
 50. The method of claim 49, wherein the magnitude of the composite signal is controlled to be less than the magnitude of the power supply voltage source.
 51. The method of claim 49, wherein the magnitude of the composite signal is controlled to be equal to the power supply voltage source.
 52. The method of claim 48, wherein the magnitude of the composite signal is controlled by a common-mode feedback circuit.
 53. The method of claim 52, wherein the common-mode feedback circuit includes an operational amplifier.
 54. The method of claim 48, further comprising providing the output of the operational amplifier to an input of a first transistor and an input of a second transistor.
 55. The method of claim 48, wherein the magnitude of the composite signal is controlled by a current source.
 56. The method of claim 55, further comprising providing a constant baseline correction current control signal to control the magnitude of the composite signal.
 57. The method of claim 48, wherein the magnitude of the composite signal is controlled by a resistor divider.
 58. The method of claim 57, wherein the resistor divider comprises a plurality of resistors, each of the resistors having a respective characteristic resistance.
 59. The method of claim 58, further comprising controlling the magnitude of the composite signal in relation to the respective resistances of each of the resistors.
 60. A method for a communications channel comprising: receiving at a first input a composite signal that includes a transmission signal component and a receive signal component; receiving at a second input a replica transmission signal; receiving at a third input an analog baseline correction current; controlling the magnitude of the analog baseline correction current to thereby control the magnitude of the composite signal; and providing at an output a receive signal which comprises the composite signal minus the replica signal, wherein the composite signal, the replica transmission signal, and the analog baseline correction current are directly connected together at a common node.
 61. The method of claim 60, further comprising providing a power supply voltage source of a predetermined magnitude.
 62. The method of claim 61, wherein the magnitude of the analog baseline correction current is controlled to control the magnitude of the composite signal to be less than the magnitude of the power supply voltage source.
 63. The method of claim 61, wherein the magnitude of the analog baseline correction current is controlled to control the magnitude of the composite signal to be equal to the magnitude of the power supply voltage source.
 64. The method of claim 60, wherein the magnitude of the analog baseline correction current is controlled by a common-mode feedback circuit.
 65. The method of claim 64, wherein the common-mode feedback circuit includes an operational amplifier.
 66. The method of claim 65, further comprising: receiving at a first input a first differential component of the composite signal; receiving at a second input a second differential component of the composite signal; receiving at a third input a common-mode voltage signal; and providing at an output a baseline correction current control signal to control an analog baseline correction current.
 67. The method of claim 66, further comprising providing the output of the operational amplifier to an input of a first transistor and an input of a second transistor.
 68. The method of claim 60, wherein the magnitude of the analog baseline correction current is controlled by a current source.
 69. The method of claim 68, further comprising providing a constant baseline correction current control signal.
 70. The method of claim 60, wherein the magnitude of the analog baseline correction current is controlled by a resistor divider.
 71. The method of claim 70, wherein the resistor divider comprises a plurality of resistors, each of the resistors having a respective characteristic resistance.
 72. The method of claim 71, further comprising providing a baseline correction current control signal that is related to the respective resistances of each of the resistors.
 73. An electrical circuit for a communications channel, comprising: means for receiving at a first input a composite signal that includes a transmission signal component and a receive signal component; means for receiving at a second input a transmission signal; means for receiving at a third input an analog baseline correction current; means for providing a receive signal comprising the composite signal minus the replica signal; and means for providing the analog baseline correction current to substantially prevent the magnitude of the composite signal from exceeding a predetermined value of an operating parameter of the electrical circuit, wherein the composite signal, the transmission signal, and the analog baseline correction current are directly connected together at a common node.
 74. The electrical circuit of claim 73, further comprising means for providing a power supply voltage source of a predetermined magnitude, wherein the operating parameter is the predetermined magnitude of the power supply voltage source.
 75. The electrical circuit of claim 73, wherein the analog baseline correction current is provided by a common-mode feedback circuit means.
 76. The electrical circuit of claim 75, wherein the common-mode feedback circuit means includes operational amplifier means.
 77. The electrical circuit of claim 76, further comprising: means for receiving a first differential component of the composite signal at a first input of the operational amplifier means; means for receiving a second differential component of the composite signal at a second input of the operational amplifier means; means for receiving a common-mode voltage signal at a third input of the operational amplifier; and means for providing a baseline correction current control signal at an output of the operational amplifier means.
 78. The electrical circuit of claim 77, further comprising means for providing the baseline correction current control signal at an input of a first transistor and at an input of a second transistor.
 79. The electrical circuit of claim 73, wherein current source means provide the analog baseline correction current.
 80. The electrical circuit of claim 79, further comprising means for providing a constant analog baseline correction current.
 81. The electrical circuit of claim 73, wherein resistor divider means provide the analog baseline correction current.
 82. The electrical circuit of claim 81, wherein the resistor divider means comprises a plurality of resistors, each resistor having a respective characteristic resistance.
 83. The electrical circuit of claim 82, wherein the analog baseline correction current is related to the respective resistances of each of the resistors.
 84. An electrical circuit for a communications circuit, comprising: means for producing a receive signal as a difference between a composite signal and a replica transmission signal, the composite signal comprising a transmission signal component and a receive signal component; means for controlling the magnitude of the composite signal; means for receiving at a first input a first differential component of the composite signal; means for receiving at a second input a second differential component of the composite signal; means for receiving at a third input a common-mode voltage signal; and means for providing at an output analog baseline correction current control signal, wherein the composite signal, the replica transmission signal, and the analog baseline correction current control signal are directly connected together at a common node of the producing means.
 85. The electrical circuit of claim 84, further comprising means for providing a power supply voltage source of a predetermined magnitude.
 86. The electrical circuit of claim 85, wherein the means for controlling the magnitude of the composite signal control the magnitude of the composite signal to be less than the magnitude of the power supply voltage source.
 87. The electrical circuit of claim 85, wherein the means for controlling the magnitude of the composite signal control the magnitude of the composite signal to be equal to the magnitude of the power supply voltage source.
 88. The electrical circuit of claim 84, wherein the magnitude of the composite signal is controlled by common-mode feedback circuit means.
 89. The electrical circuit of claim 88, wherein the common-mode feedback circuit means include operational amplifier means.
 90. The electrical circuit of claim 84, further comprising means for providing the output of the operational amplifier means to an input of a first transistor and an input of a second transistor.
 91. The electrical circuit of claim 84, wherein the magnitude of the composite signal is controlled by current source means.
 92. The electrical circuit of claim 91, further comprising means for providing a constant analog baseline correction current control signal to control the magnitude of the composite signal.
 93. The electrical circuit of claim 84, wherein the magnitude of the composite signal is controlled by resistor divider means.
 94. The electrical circuit of claim 93, wherein the resistor divider means comprises a plurality of resistors, each of the resistors having a respective characteristic resistance.
 95. The electrical circuit of claim 94, further comprising means for controlling the magnitude of the composite signal in relation to the respective resistances of each of the resistors.
 96. An electrical circuit for a communications channel, comprising: means for receiving at a first input a composite signal that includes a transmission signal component and a receive signal component; means for receiving at a second input a replica transmission signal; means for receiving at a third input an analog baseline correction current; means for controlling the magnitude of the analog baseline correction current to thereby control the magnitude of the composite signal; and means for providing at an output a receive signal which comprises the composite signal minus the replica signal, wherein the composite signal, the replica transmission signal, and the analog baseline correction current are directly connected together at a common node.
 97. The electrical circuit of claim 96, further comprising means for providing a power supply voltage source of a predetermined magnitude.
 98. The electrical circuit of claim 97, wherein the means for controlling the magnitude of the analog baseline correction current is controlled to be less than the magnitude of the means for providing a power supply voltage source of a predetermined magnitude.
 99. The electrical circuit of claim 97, wherein the means for controlling the magnitude of the analog baseline correction current is controlled to be equal to the magnitude of the means for providing a power supply voltage source of a predetermined magnitude.
 100. The electrical circuit of claim 97, wherein the magnitude of the analog baseline correction current is controlled by common-mode feedback circuit means.
 101. The electrical circuit of claim 100, wherein the common-mode feedback circuit means includes operational amplifier means.
 102. The electrical circuit of claim 101, further comprising: means for receiving at a first input a first differential component of the composite signal; means for receiving at a second input a second differential component of the composite signal; means for receiving at a third input a common-mode voltage signal; and means for providing at an output a baseline correction current control signal.
 103. The electrical circuit of claim 102, further comprising means for providing the output of the operational amplifier to an input of a first transistor and an input of a second transistor.
 104. The electrical circuit of claim 96, wherein the magnitude of the analog baseline correction current is controlled by current source means.
 105. The electrical circuit of claim 104, further comprising means for providing a constant analog baseline correction current control signal.
 106. The electrical circuit of claim 96, wherein the magnitude of the analog baseline correction current is controlled by resistor divider means.
 107. The electrical circuit of claim 106, wherein the resistor divider means comprises a plurality of resistors, each of the resistors having a respective characteristic resistance.
 108. The electrical circuit of claim 107, further comprising means for providing a baseline correction current control signal that is related to the respective resistances of each of the resistors. 